Martin Luther (1483–1546) was a 34-year-old Augustinian friar and professor of biblical theology at the small new University of Wittenberg in autumn 1517. The proximate trigger of his Ninety-Five Theses was the substantial Dominican preacher Johann Tetzel, who had been authorised by Archbishop Albrecht of Mainz to sell indulgences in eastern Germany to fund the substantial reconstruction of St Peter’s Basilica in Rome. Tetzel’s pitches — substantively reported in contemporary German pamphlets — had crossed from theological-disciplinary practice into substantively crude commercial transaction.
Luther’s response was a substantial Latin academic disputation document — 95 numbered theological propositions, each substantively a substantively short logical-disputational position, the whole intended to be argued in a formal university debate. The document was substantively academic in form, but its substantive content substantively challenged the papal authority to issue indulgences at all.
What he actually did on 31 October
The single confirmed Luther action on 31 October 1517 is that he sent the Ninety-Five Theses by letter to Archbishop Albrecht of Mainz. The substantively letter survives. The substantively theses were enclosed as a substantively appended document. Luther’s substantively cover letter substantively requested that the archbishop discipline Tetzel and substantively substantively review the Catholic-doctrinal basis of the indulgence sales.
The Wittenberg Castle Church door story is a separate tradition. The single source is Luther’s colleague Philip Melanchthon, who substantively wrote about the nailing in the substantively preface to Luther’s collected works — published in 1546, twenty-nine years after the event. Melanchthon substantively had not personally been at Wittenberg in October 1517 (he was substantively then at Tübingen and substantively did not arrive at Wittenberg until August 1518). His substantively account is substantively second-hand and substantively was substantively written after Luther’s death.
Modern Reformation scholarship is divided. The Iserloh thesis (Erwin Iserloh, 1962) argued that the nailing substantively never happened — that Luther substantively sent the theses privately to Mainz and that the public-confrontation narrative substantively was a substantively retroactive Reformation invention. The substantively traditionalist position substantively defends the nailing as substantively historically plausible (Wittenberg academic-disputation announcements substantively were posted on the church door as a standard procedural notice; the substantively practice substantively was substantively unremarkable in context).
Why it does not matter
Whether Luther nailed the theses or only mailed them, the subsequent course of the Reformation was substantively the same. The Ninety-Five Theses was substantively reprinted at Leipzig, Nuremberg, and Basel by January 1518 — substantively through the new Gutenberg printing-press infrastructure that substantively distributed substantively academic-theological material across substantively German-speaking Europe within substantively weeks. The substantively papal response substantively came in summer 1518; Luther was substantively excommunicated in January 1521; the substantively Diet of Worms in April 1521 substantively substantively cemented the substantively political-confessional split that would substantively substantively define European Christianity for the subsequent five centuries.
The nail was substantively either there or substantively not. The Reformation happened either way.